For 15.6 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/network:messaging:matrix/15.6/network:messaging:matrix.repo
zypper refresh
zypper install python-immutabledict
For 15.5 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/network:messaging:matrix/15.5/network:messaging:matrix.repo
zypper refresh
zypper install python-immutabledict
For 15.4 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/network:messaging:matrix/15.4/network:messaging:matrix.repo
zypper refresh
zypper install python-immutabledict